This invention relates to polymeric agents useful for thickening aqueous systems including latex paints and other aqueous coating compositions. This invention also relates to mixtures of such thickening agents.
For many years additives have been sought for aqueous systems to increase viscosity and to maintain viscosity at required levels under specified processing conditions and end use situations. For this reason, such additives are commonly called "thickeners". Thickeners are useful in oil well drilling and flooding fluids, fire-fighting foams and fluids, detergents, leather pastes and finishes, adhesives, pharmaceuticals, agricultural formulations, latex paints, printing inks, cosmetics, paper coatings, and emulsions of all kinds. The list of applications and auxiliary properties of thickeners is virtually endless.
Among the many well-known thickeners may be mentioned natural products such as the alginates, casein, gum karaya, locust bean gum and gum tragacanth and modified natural products such as the cellulosics, including methyl cellulose, hydroxyethyl cellulose and hydroxypropylmethyl cellulose. Totally synthetic thickeners are also available such as carboxy vinyl ether copolymers, acrylic polymers and maleic anhydridestyrene copolymers. However, all of the known thickeners have deficiencies.
Cellulose ethers, such as described in Glomski et al., U.S. Pat. No. 3,769,247, are often effectively used as thickeners for latex paints. However, solid, water-soluble polymers derived from cellulose and other natural products are becoming increasingly expensive to produce because of high capital, energy, and waste control costs.
Alkali-soluble latex copolymers have been known for some time. Thus, Hager and Martin, U.S. Pat. Nos. 3,003,987 and 3,070,561 and Miller, U.S. Pat. No. 3,081,198 describe copolymers of acrylic and methacrylic acids and esters which may be thickened by replacing a portion of the hydrogen ions of the copolymer carboxyl groups with ammonium or alkali metal ions. Other types of polymeric thickeners are disclosed by Junas and LaTorre, U.S. Pat. Nos. 3,652,497 and 3,708,445; Zimmerman, U.S. Pat. No. 3,657,175; Chang and McDowell, U.S. Pat. No. 3,891,591; and Gibson, U.S. Pat. No. 4,003,870. All contain various carboxylic acid groups which can be solubilized in water by neutralization with a water-soluble base. However, to date this technology has had limited impact on major markets for water-soluble polymeric thickeners.
More recently, Evani and Corson developed, as described in U.S. Pat. No. 4,008,202 and related patents, a solid styrene-maleic anhydride-vinylbenzyl ether terpolymer soluble at high pH and useful as a thickener for aqueous solutions. In spite of excellent rheology, stability problems and cost have limited its use as a paint thickener. Further improvements in this technology are clearly desirable.
A very recent advance in thickener technology, described by Emmons and Stevens in U.S. Pat. No. 4,079,028, involves the use of low molecular weight polyurethanes which have hydrophobic groups interconnected by hydrophilic polyether groups. While these materials do provide a thickening action, paint formulations made therefrom nonetheless exhibit undesirable sagging. Further, paint formulations made therefrom undergo syneresis (separation of solid and liquid components).